The long range objective of the project is the successful use, in the therapy of cancer, of high-dose methotrexate and the methotrexate-cleavage enzyme, a carboxypeptidase, synthesized by a Flavobacterium sp. It is hypothesized that the appropriately scheduled administration of the enzyme after methotrexate would lower toxic drug levels rapidly and effectively, thereby hastening the recovery of DNA synthesis of sensitive cell renewal tissues to prevent cytotoxicity. The efficiency of essentially homogeneous carboxypeptidase preparations reflecting 2000-fold purification is under evaluation in the murine leukemia L1210 system. The overall approach to the assessment of the enzyme is relevant to the pharmacokinetics of high-dose methotrexate and considers the relative effects of methotrexate and carboxypeptidase on plasma folate levels. Studies will explore possibilities of biochemical differences between leukemia cells and the small intestine that may influence selective rescue of sensitive host tissue.